What about a solar greenhouse heater
[www.reuk.co.uk/Solar-Greenhouse-Heat-Sink.htm]www.reuk.co.uk/Solar-Greenhouse-Heat-Sink.htm[/url]
You'd have to put a cubic meter of a heat sore into the van,.. unless you could think of a way to transfer the heat to you water tank?
Before I got the hot system, I just used to fill 2 25 litre drums with boiling water and throw them in the back of the van late at night with an old quilt over them. They'd be nearly stone cold in the morning, but nothing was frozen.
Seriously though,.. for the £100 or so involved, get a gas heater,.. you'd prob spend half that anyway on an oil filled rad and a long extension lead!